A campaign needs a minimum of one Creative to be eligible for ad serving.
In Passendo, Creatives are assigned to a format in the same manner that Placements are. This ensures that a Placement only shows ads that you have clearly marked to fit the specific format. In other words, you will not be able to place a creative assigned to a 300x250 format on a placement that is set to 600x250. Furthermore, you can only create creatives on a campaign that matches one of the Placement formats currently created in your account.
You can have as many Creatives within a campaign as you like. You can even make a mix of native and display creatives on the same campaign. Depending on how the campaign is booked (covered in detail here: Bookings ), the system can rotate all Creatives of a format on all Placements in the same format, allowing for very flexible testing and optimisation on CTR, CR, eCPM, etc. on the Creative level.